Throughout the specification (including in the claims) the terms "video data" and "video signal" are used to denote any of the broad class of image signals indicative of pixels of an image, including analog image signals and digital image signals.
A video mixer selectively combines pixels of a first video signal with pixels of a second video signal, responsive to a video mixer control signal, to produce pixels of a composite video signal. The composite video pixels have values v.sub.h,v =(1-.alpha..sub.h,v)a.sub.h,v +(.alpha..sub.h,v)b.sub.h,v. "h" and "v" represent the pixel horizontal and vertical coordinates, respectively. .alpha..sub.h,v represents the video mixer control signal values. Each video mixer control signal value .alpha..sub.h,v has a value in the range from 0 to 1.0, which may be represented as a fixed point fractional value. a.sub.h,v represents the pixel values of the first video signal and b.sub.h,v represents the pixel values of the second video signal.
In typical embodiments, the first and second video signals are color video signals whose color components (e.g., red, green, and blue components, or luminance and chrominance components) are multiplexed according to the well known CCIR 601 standard (or the well known SMPTE/EBU recommended practice known as RP125).
A wipe surface is a data stream representing a geometric shape from which the video mixer control signal .alpha. is generated. That is, the data value of a point on the wipe surface determines the video mixer control signal value .alpha..sub.h,v for a pixel h,v of the composite image which corresponds to the point on the wipe surface. A wipe surface may be formed, for example, by a function z(h, v), where h and v are the horizontal and vertical coordinates of a display device, such as a video raster, on which the composite image is to be displayed. A wipe surface 20 is shown in a constant v cross-section, i.e. as a function of h only, in FIG. 1A, and the corresponding video mixer control signal .alpha., generated from the wipe surface 20, is shown in FIG. 1B.
.alpha..sub.h,v 's for pixels h,v, in the composite video image, which correspond to points on the wipe surface below a lower threshold plane 22 are set to 0.0. That is, v.sub.h,v for these pixels is comprised of a.sub.h,v only. .alpha..sub.h,v 's for pixels h,v, in the composite video image, corresponding to points on the wipe surface above an upper threshold plane 24 are set to 1.0. That is, v.sub.h,v for these pixels is comprised of b.sub.h,v only. The upper threshold surface is above the lower threshold surface at all points. An upper threshold plane may be generated, for example, by adding some constant (delta) to points on a lower threshold plane, such as is shown in FIG. 1A.
.alpha..sub.h,v 's for pixels h,v, in the composite video image, which correspond to points on the wipe surface above the lower threshold surface 22 and above the upper threshold surface 24 are set to a value between 0.0 and 1.0. That is, v.sub.h,v for these pixels is comprised of a combination of a.sub.h,v and b.sub.h,v. The closer a point on the wipe surface 20 between the lower threshold surface 22 and the upper threshold surface 24 is to the lower threshold surface 22, the closer the corresponding .alpha. value is to 0.0. Similarly, the closer such a point on the wipe surface 20 is to the upper threshold surface 24, the closer the corresponding .alpha. value is to 1.0.
Wipe pattern generation systems are comprised of basic circuit elements. For example, analog circuit implementations have employed operational amplifiers, integrators, two and four quadrant multipliers, and voltage comparators. Digital circuit implementations have employed logic elements such as multipliers, accumulators, digital switches, and comparators.
A conventional wipe surface generation system employs a fixed combination of basic circuit elements to generate an a priori determined wipe surface. A disadvantage of the conventional wipe surface generation system is its lack of flexibility, since the generated wipe surface is a priori determined.